JPH0267608A - Alternating current constant voltage device - Google Patents

Abstract

PURPOSE:To prevent plural thyristors from being simultaneously turned on and to instantaneously switch an output tap by using the accumulation energy of a capacitor or the induced energy of a reactor for the commutation of a bidirectional thyristor. CONSTITUTION:The voltage transformer 2A with a tap is provided with an auxiliary tap Ts and capacitors C1 to Cn for accumulating commutation energy are connected between respective output taps T1 to Tn. At the time of switching the output taps based upon the detection of variation in the voltage of an AC power supply 1, two semiconductor switches Qa, Qb for positive and negative commutation are alternately turned on and driven in accordance with the positive and negative states of the power supply simultaneously with the stop of ignition control of the ignited thyristors S1 to Sn and respective thyristors are reversely biased by commutation energy based upon the accumulation energy of respective capacitors to instantaneously execute forced commutation deignition. Thereby, the generation of no control of output AC due to the simultaneous ON of the thyristors can be prevented and constant voltage AC can be supplied to a load.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an AC constant voltage device that supplies constant voltage AC to a load regardless of voltage fluctuations of an AC power supply.

For example, Japanese Patent Application Laid-Open No. 53-81928 (G
O5F 1/20) combines a tapped transformer and multiple bidirectional thyristors, and the transformer's primary IJ
An AC voltage constant device is described that selectively selects and fires each thyristor based on the detection of voltage fluctuations of an AC power source, and switches and supplies the output of each tap of a transformer to a load.

By the way, when switching the tap output by firing the valley thyristor, in order to prevent damage due to simultaneous ON of multiple thyristors, the target thyristor must be switched on after commutation and extinguishing of the thyristor that has already been turned on. It is necessary to shift to thyristor firing control.

In the case of the AC voltage regulator described in the above-mentioned publication, the characteristic of the thyristor that naturally commutates due to the zero crossing of the conducting current is utilized, and when switching the tap output, there is an error in the tap switching control, and the thyristor is turned on by the pre-ignition. The supply of the gate signal for ignition to the thyristor in the target tap is cut off, the thyristor is commutated and extinguished by natural commutation, and then the thyristor at the target tap is ignited in synchronization with the zero-crossing of the voltage of the AC power supply. All gate signals are supplied.

[Problem to be solved by the invention] When extinguishing the arc of a thyristor by natural commutation as described above,
The time required to extinguish the arc depends on the cycle (frequency) of the AC power supply, and when the load current flowing through the thyristor in the on state is in a delayed phase, the thyristor commutates and extinguishes the arc with a delay from the zero cross of the voltage. .

Therefore, it takes approximately one AC power cycle (
In order to reliably prevent multiple thyristors from turning on simultaneously, which requires a long time (approx. It is necessary to perform this after the long arc extinguishing period has elapsed.

As a result, the tap output cannot be switched quickly, and the output AC remains uncontrolled for a long period of at least one cycle, during which the load is adversely affected by excessive or insufficient voltage application. There is a point.

3. The above publication only describes that all thyristors of the target tap are fired based on zero-cross detection of the voltage of the AC power supply and in synchronization with the zero-crossing of the voltage of the current power supply.
In this case, the tap output cannot be quickly switched, and
Because multiple thyristors may turn on at the same time due to the phase delay of the load current mentioned above, the voltage between the taps is regulated, and in order to perform multi-stage switching of the output, the transformer winding configuration and tap switching are required. Control needs to be complex and special.

SUMMARY OF THE INVENTION An object of the present invention is to provide an AC constant voltage device that can reliably prevent a plurality of thyristors from being turned on simultaneously and can quickly switch tap outputs.

[Means to solve the problem]

Means for achieving the above object will be explained below with reference to FIGS. 1 and 3, which correspond to embodiments.

As shown in FIG. 1, the present invention includes a plurality of output taps (TI) on a transformer (two) connected to an AC power source (1).

(Tm), .

(Sri, -(Sn-+), (Sn) each -it
At the same time, each of the thyristors (Sl) to (Sn
) is connected to the output terminal (3), and by tap switching control based on the detection of the voltage fluctuation of the power source 11), the firing control of the already fired thyristano? In the AC constant voltage device, the AC constant voltage device supplies the constant voltage AC to the load connected to the output terminal (3), and shifts to the firing control of the thyristor of the target output tap after commutation and extinction of the thyristor. Transformer (
The highest voltage auxiliary tap (Ts) provided in 2A), the auxiliary tap (Ts) and each output tap (T□)
A plurality of capacitors (C1) for commutation energy storage provided between ~(Tn) and 1: (negative), . . .
, (cn-+).

(Cn), the auxiliary tap (Ts), and each of the thyristors (S,)
~ (Sn) Two semiconductor switches for positive and negative commutation connected in parallel with mutually opposite polarity between the connection point at the other end, and at the same time the firing control of the already fired thyristor is stopped. Depending on whether the voltage of the type is positive or negative, both the switches are selectively instantaneously turned on, and the thyristors (Sl ) to (Sn) are all equipped with a commutation control circuit for forced commutation and arc extinguishing.

Moreover, as shown in FIG.
-('I's), ・=-(Tn-+), (Tn)
? A plurality of bidirectional thyristors (81) are provided for each of the output taps (T1) to (Tn).

(Ss), ・=, (Sn-1), (Sn) are connected at one end of each, and each of the thyristors (Ss)
The other ends of t) to (Sn) are connected to the output terminal (3), and the firing control of the already fired thyristor is stopped by tap switching control based on the detection of the voltage fluctuation of the power supply (1). After the commutation of the thyristor extinguishes, the thyristor of the target output tap is controlled to start, and the AC constant voltage device supplies the constant voltage AC to the load connected to the output terminal (3);
A secondary winding (L, ) is inserted between the connection point of the other end of each of the thyristors (Sl) to (Sn) and the output terminal (31).

The intermediate tap (Tm) of the primary winding # (Ll) is provided between a grounded reactor (4) for injecting commutation energy, and each of both ends of the DC power supply and the primary winding (L, ). two semiconductor switches for positive and negative commutation, and at the same time as stopping the firing control of the already fired thyristor, selectively switching both the switches according to the positive or negative voltage of the recurrent power source fi+; Instantly turn on and drive the secondary winding FA CLl)
Each of the thyristors (Sl) to (Sn)
A technical measure is taken to include a commutation control circuit for forced commutation arc extinguishing.

[Effect]

Because it is configured as above! ! In the case of Figure 81, the auxiliary tap (Ts) and the valley output tap (TI) ~ (Tn
) based on the voltage between each capacitor (C
1) to (Cn) are charged to reverse polarity according to the positive and negative of the power supply flll, and when the voltage of the power supply 11) fluctuates,
At the same time as the commutation control circuit stops the firing control of the thyristor that has already fired, the two semiconductor switches are selectively instantaneously turned on depending on the positive or negative polarity of the power supply +11 at that time. The thyristor is an auxiliary tap.

It is reverse biased by commutation energy based on the stored energy of the capacitor, and is immediately extinguished by forced commutation.

Therefore, the commutation extinction of the already fired thyristor is caused by the power supply f1
It can be performed instantaneously over several hundred microseconds depending on the characteristics of the thyristor without depending on the cycle of 1, and after stopping the firing control of the already fired thyristor, it immediately shifts to the firing control of the thyristor of the target output tap. This prevents a plurality of thyristors from turning on simultaneously and allows instantaneous tap output switching.

In addition, in the case of Fig. 3, when the voltage of the power supply il+ fluctuates, at the same time as the firing control of the already fired thyristor stops,
The two semiconductor switches are selectively instantaneously turned on by the commutation control circuit, and at this time, the current direction of the primary winding (Lm) of the reactor (4) is reversed by turning on each of the bidirectional conductor switches. Along with the primary winding (Ll)
The already fired thyristor is reverse biased by the induced energy of the secondary winding (L) whose polarity is reversed depending on the current direction of the secondary winding, and the thyristor is immediately extinguished by forced commutation.

Therefore, as in the case of Fig. 1, the commutation extinction of the already fired thyristor takes several hundred microseconds depending on the characteristics of the thyristor.
This can be done instantaneously, preventing multiple thyristors from turning on at the same time, and instantly switching the tap output.

〔Example〕

An embodiment will be described with reference to FIGS. 1 to 3.

(One Example) First, FIG. 1 and FIG. 2 showing one example will be described.

As shown in Figure 1, a single-turn transformer (2A) is connected to the AC power supply (1) of a 50V or 60V system, and at this time, the main transformer (2A) has a designated tap as the primary side tap. An input tap (Ti) is provided at the turn position, and
In order to perform n-stage switching of tap output, n output taps (Ts) and (Tm) are used as secondary side taps.

..., (Tn-+), (Tn) are provided.

In addition, in order to perform forced commutation arc extinguishing described later, a transformer (2A)
The highest voltage auxiliary tap (Ts) is provided at one end of the winding.

Based on the power supply voltage applied between the input tap (Ti) and the input side common terminal (Ti)' at the other end of the binding,
Each output tap (T1) to (Tn), auxiliary tap (Ts
) and the output side common terminal (To) at the other end of the winding, a voltage according to the tap position is generated. For example, the output tap (T1) at the predetermined turn position has the same voltage as the power supply voltage. Occur.

Furthermore, each output tap (T1) to (Tn) is equipped with n three-terminal type bidirectional thyristors (Sl
), (Sri, "", (Sn-+), (Sn) are connected at one end, and each thyristor (Sl)
The other end of ~(Sn) is connected to one output terminal (3) via a reactor (5) for blocking the output of commutation energy, which will be described later, and the alternative of each thyristor (S, ) ~(Sn) is Based on the ignition control, ・Power terminal (3), common terminal ÷≠(To
AC voltages of the output taps (T1) to (Tn) are switched and output between the other output terminal (3) connected to the output terminal (3).

Between the S1 output terminals +31 and +31', a noise absorbing capacitor (Ca) and a resistor (R, a), which together with the reactor (5) form an AC absorber, are provided.

In addition, the auxiliary tap (Ts) and each output tap (T1) ~
(Tn), there are n capacitors (C1) for commutation energy storage, (Cri, -, (Cn-+), (
Cn) respectively, and the auxiliary tap (Ts
) and each of the output taps (Tl) to (Tn), each capacitor (C8) to (Cn) is charged, and at this time, each capacitor (C1) to (Cn)
The charging polarity of is reversed in synchronization with the power supply (1).

Furthermore, when switching the tap output, the auxiliary tap (Ts
) and each capacitor (C1) to (Cn) to reverse bias each thyristor (S,) to (Sn), the auxiliary tap (Ts) and each thyristor (S,) to (Sn) Two NPN transistors (Qa) and (Qb) are connected in parallel with opposite polarities between the connection point at the other end and serve as two semiconductor switches for positive and negative commutation. ing.

2. Is the collector of the transistor (Qa) a diode (Da) for preventing backflow? It is connected to the auxiliary tap (Ts) via the auxiliary tap (Ts).

In addition, the emitters of transistors (Qa) and (Qb),
A diode (Dc) for surge absorption is installed between the collectors.

(Dd) are provided respectively.

On the other hand, detection of voltage fluctuations of the power supply 111 and each thyristor (
At points Sl) to (Sn), in order to control arc extinction, the power supply f1
1 is detected by the detection transformer (6), and the detection output of the secondary side of the transformer (6) is detected by the DC conversion circuit (7),
It is input to the synchronous pulse generation circuit [8] and converted to the conversion circuit (7).
A DC voltage based on the AC voltage of the power supply +11 is output from the generating circuit (8) to the integrating circuit (9), and positive and negative synchronous pulses of the voltage of the power supply +11 are output from the generating circuit (8) to the integrating circuit (9) from one cycle. A sufficiently short interval, e.g. 1/
Output every 4 cycles.

Then, each one of the conversion circuits (7) is
/4 cycles of output integration are repeated, and
The integrated voltage of the integrating circuit (9) is output to the determination circuit αQ.

Furthermore, the judgment circuit OQ calculates the error between the integrated voltage of the integrating circuit (9) and the reference voltage of the detection reference voltage (Fukayama), and the judgment circuit 0a sends it to the decoder circuit O2 as the detection voltage of the voltage fluctuation of the power supply il+. , a differential voltage between the integrated voltage and the reference voltage is output.

Then, the decoder circuit Q21 converts the input differential voltage into a valley output tap (TI) according to the polarity and magnitude of the voltage.
) to (Tn) is converted into n-bit selection data that alternatively selects one of them, and the selection data is input to the tap switching command circuit (131).

Furthermore, based on the selection data input every 1/4 cycle, the output taps (T1) to (
The presence or absence of switching of thyristors (Sl) to (Sl) is detected from the command circuit 03 based on the detection result.
The gate signal for ignition is alternatively output to gate +01 of n), and the constant voltage AC of the rated voltage of the output terminal (power supply 11 between 31 and f31') is output. (S1) to (Sn) are selectively controlled to turn on;
A constant voltage AC is supplied to the load between the output terminals [31 and +3r.

For example, if the voltage of the power supply [+1] decreases and fluctuates during firing control of the thyristor (Sl), based on the output of the integrating circuit (91), within an extremely short time of 1/4 cycle,
A voltage drop is detected by the determination circuit αO, and at this time,
Based on the selection data of the decoder circuit i1Z, the command circuit (131) switches the tap output, for example, the output tap (
'rI) to the output tap ('l'n) is detected.

By the way, when the switching of the tap output is detected, the command circuit 03 cuts off the gate signal of the fired thyristor (S,) and at the same time outputs a commutation command signal to the commutation drive circuit 04.

In addition, the drive circuit I performs output switching based on the positive and negative synchronizing pulses of the generating circuit (8), and uses the commutation command signal from the drive circuit 0 period as a positive side and negative side ON signal to turn on a transistor (Qa), Ha of (Qb), (Bb)
It is alternatively output instantaneously (several 100 μsec).

Therefore, when tap switching occurs in the positive half cycle of the voltage of the power supply fl'l, the command circuit QJ, the drive circuit +
By the commutation control circuit formed by 141, the thyristor (Sl) of the corpse ignition is activated based on the interruption of the gate signal? ! i
At the same time as II stops, transistor (Qa) 5 is turned on instantly.

At this time, the energy stored in the auxiliary tap (Ts) and capacitor (C+) is transferred to the diode (Da) and transistor (Q
The thyristor (S,) is reverse-biased and the commutation is forcibly extinguished, and the load current is in a lagging phase. Also, the thyristor (Sl) is immediately turned off.

Furthermore, when the output of the ON signal of the drive circuit 041 is completed, the command circuit (+3 outputs a gate signal to the thyristor (Sn) of the target output tap (Tn) specified by the selection data, and 100 off from
After a delay of μsec, the control immediately shifts to ignition control of the thyristor (Sn), and turns on the thyristor (Sn) (H) to switch the tap output.

Therefore, the process from detecting a voltage drop to completing switching of the tap output can be done in an instant of several hundred microseconds, and moreover, after the previously fired thyristor (Sl) is extinguished by forced commutation extinguishing, the target output can be achieved. Thyristor (Sn) with tap (Tn)
is fired, and the thyristors (Sl) and (Sn,) are not turned on at the same time.

2. When switching of the tap output occurs during the negative half cycle of the 'f pressure of the power supply Il+, the transistor (Qb) turns on, and based on the reverse bias similar to the positive half cycle, the already fired signal is switched on. The thyristor is forced commutated and extinguished.

In addition, the commutation energy flows out to the load through the reactor 15)
The number 1 caused by commutation quenching 1 ignition is blocked by
00μsec noise is caused by capacitor (Ca), resistor (R
a) absorbed by.

Furthermore, it goes without saying that the switching direction of the output tap changes as the voltage of the power source (1) increases or decreases.

By the way, the capacitances of capacitors (C1) to (Cn) are
(j=1...-mouth), each capacitor (C3
) ~ (Cn) The energy stored in each Jj (=
, CVj [Vj is the auxiliary tap (Ts), the voltage between each output tap (Tt) and (Tn)] is set to each thyristor (
In order to make the capacitances Cj of each of the capacitors (C1) to (Cn) so large that they can be forcedly commutated and extinguished, the capacitances Cj of the capacitors (C1) to (Cn) are weighted in stages.

In addition, when setting the output voltage to 90 to 1 oov with respect to the voltage of 100 to 40 V of the power supply 11+, the output tap (
The number of turns of the auxiliary tap (Ts) is 1. It is set to about IN.

Further, the waveforms of the output AC voltage and current when the voltage of the power source I11 increases and fluctuates are as shown in FIG. 2(a), for example.

(As shown in the solid lines of bl and fat in the same figure,
(At 8 in bl, the broken lines indicate the waveforms of abnormal voltage and abnormal current based on fluctuations, and τ indicates the tap output switching period of several 100 μsec.

Of course, various transformers such as a single-turn transformer, a transformer having primary and secondary windings, etc. may be used as the transformer (two people).

Furthermore, the method for detecting voltage fluctuations of the power source il+ may be different from the embodiment.

(Other examples) Next, FIG. 3 showing another embodiment will be explained.

8 in Fig. 3, the difference from Fig. 1 is that it is used for the transformer (2B) ' without the auxiliary taps ('L's) in Fig. 1, and the auxiliary taps (''s) in Fig. 1 are *Capacitor (
Instead of the stored energy of C1) to (Cn), the energy induced in the reactor 14) for injecting commutation energy is used for forced commutation arc extinguishing of each thyristor (Sl) to (Sn).

The reactor +41 has a secondary winding (L,) inserted between the other end of each thyristor (S,) to (Sn) and the output terminal (3), and a primary winding m (Lj). The middle tap (Tm) is grounded.

On the other hand, each thyristor (S□) when switching the output tap
In order to obtain energy for reverse bias of ~(Sn), a small and small capacity power transformer α61. Rectifier diode (De
), (1) f) changes the AC of the power supply (!),
In addition to being full-wave rectified, the diode (De), (D
The rectified output of f) constantly charges a large-capacity smoothing and integrating capacitor (cb) that forms a DC power supply.

Also, the primary winding of the charging energy of the capacitor (cb)? t
Switch the direction of power feeding to s(LJ) and determine the polarity of the induced energy in the secondary winding (L,). In order to switch according to the positive or negative voltage of the power supply f11, the capacitor (cb) and the primary winding (Ll
), two NPN transistors (Q
C) and (Qd) are each inserted.

3. The collector of the transistor (Qs, (Qd) is connected to the capacitor (Cb) via the connecting resistor (Rb)', and the emitter is a diode (Dg) for preventing backflow.

(Dh) one end of the primary winding m (Ll) through each;
connected to each other end.

In addition, the base terminals (Be) and (Bd) K of the transistors (QC) and (Qd) are the transistors (
Qa).

Similarly to the base terminals (Ha) and (Bb) of (Qb), the positive side and negative side ON signals of the drive circuit a41 are respectively input.

Therefore, based on the voltage fluctuation of the power supply (for example), if switching from the output tap (Tj) 7JTh to the output tap (Tn) occurs in the positive half cycle of the voltage of the power supply 11, the fired thyristor (S At the same time as the gate signal to , ) is cut off, the transistor (QC) is instantaneously turned on by the positive ON signal of the drive circuit Q4, and at this time, based on the open polarity of the primary winding (L, ), Secondary winding (
The polarity indicated by ■ and e in the figure, that is, the thyristor (
Induced energy of polarity with the connection point of Sl) to (Sn) as positive is generated, and this induced energy is injected into the thyristor (S,) as commutation energy, and the thyristor (Sl) is reverse biased and forced commutation occurs. The arc is extinguished and turns off immediately.

After the commutation of the thyristor (S□) is extinguished, the firing control of the thyristor (Sn) of the target output tap (Tn) is performed in the same manner as in Fig. 1, and the switching of the output tap is completed. do.

During the negative half cycle of the voltage of the power supply CO, the transistor (Qd
) is turned on, and the second volume #! The polarity of the induced energy of (Ll) is opposite to that of the positive half cycle.

Incidentally, the capacitance of the capacitor (cb) and the like are set in advance so that the commutation of each thyristor (Sl) to (Sn) can be reliably extinguished regardless of voltage fluctuations of the power supply (11).

In the case of Fig. 3, thyristors (Sl) to (Sn
) is carried out based on the charging energy of the capacitor (cb), and as in the case of Fig. 1, the commutation of the capacitor (cb)
1) There is no need to adjust the weighting of the capacities of ~(Cn), and there is no need to provide the auxiliary tap (Ts) +i in Figure 1 on the transformer (2B), so there is no need to adjust the weighting. A device is formed.

2. As a DC power source, is the power supply il+ and the rectified output of an AC power source in a separate system, or a battery power source? May be used.

〔Effect of the invention〕

Since the present invention is configured as described above, it produces the effects described below.

In addition to providing an auxiliary tap on the transformer, a capacitor for storing commutation energy is provided between the auxiliary tap of the transformer and each output tap, and when switching the output tap based on the detection of voltage fluctuations of the AC power supply. At the same time as the firing control of the already fired thyristor is stopped, each thyristor is reverse biased by the commutation energy that is commutated in a positive or negative manner according to the positive or negative voltage of the AC power supply, and the fired thyristor is By forcing the commutation to extinguish, it is possible to instantly turn off the ignited thyristor without complicating the transformer winding configuration or tap switching control, and without using a power source to utilize all of the commutation energy. After commutation extinguishing, it is possible to immediately shift to firing control of the thyristor of the target tap, and even if the load current is in a delayed phase, the already fired thyristor can be reliably turned on by forced commutation extinguishing. Since the current is extinguished, it is possible to prevent multiple thyristors from turning on at the same time and switch output taps instantly, and prevents uncontrolled output AC based on output tap switching, allowing constant voltage AC to be applied to the load. can be supplied.

In addition, a reactor for injecting commutation energy is installed, and when switching the output tap based on the detection of voltage fluctuations of the AC power source, the positive and negative voltage of the AC power source can be changed at the same time as the firing control of the already fired thyristor is stopped. In response to this, the two semiconductor switches for positive and negative commutation are selectively turned on instantaneously, and based on the direct current of the DC power supply, the positive voltage of the AC power supply is applied to the secondary winding of the injection reactor.

Generates induced energy whose polarity is reversed depending on the negative
By reverse-biasing each thyristor using electromotive energy and forcibly commutating and extinguishing the already-fired thyristor, it is possible to avoid complicating the winding configuration of the transformer and tap switching control, and furthermore, it is possible to It is possible to inject induced energy as commutation energy into each thyristor without any adjustment, instantaneously commutate and extinguish the already fired thyristors, and immediately shift to firing control of the thyristor of the target tap. IEf
Similar to the case where all the auxiliary taps are used in the E device, the simultaneous on-state of a plurality of thyristors is completely prevented and the output taps can be switched instantly.

[Brief explanation of the drawing]

FIG. 1 is a wiring diagram of one embodiment of the current constant voltage device of the present invention.
FIG. 2 fan and tbl are output AC voltage and current waveform diagrams for explaining the operation of FIG. 1, and FIG. 3 is a wiring diagram of another embodiment of the present invention. 1]) AC power supply, (2A), (2B)...Transformer, (3)...Output terminal, +41-...Reactor for commutation energy injection, (T1)~('l”n)...
Output tap, (Ts)... Auxiliary tap, (C1) ~ (
Cn)... Capacitor for commutation energy storage, (S,
)~(Sn)...Bidirectional thyristor. Agent Patent Attorney Fuji 1) Ryutabeも8 1..., modenki pay...0 No. 33°...dekatameyo 7...1 friend, chiryo ω tin 8 - u ■ pile Dris°+1-2klEIX+9...4
10... Town saw 12... Decoater SBa 13... Ri, scrap metal εB ship 14...
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Claims (2)

[Claims] (1) A transformer connected to an AC power source is provided with a plurality of output taps, one end of each of a plurality of bidirectional thyristors is connected to each of the output taps, and the other end of each of the thyristors is connected to an output terminal. , by tap switching control based on the detection of the voltage fluctuation of the power supply, stopping the firing control of the already fired thyristor, and shifting to the firing control of the thyristor of the target output tap after commutation extinguishing of the thyristor; In the AC constant voltage device that supplies constant voltage AC to a load connected to the output terminal, an auxiliary tap with the highest voltage provided on the transformer, and an auxiliary tap provided between the auxiliary tap and each of the output taps, respectively. a plurality of capacitors for storing commutation energy; and two semiconductor switches for positive and negative commutation connected in parallel with mutually opposite polarities between the auxiliary tap and the connection point at the other end of each of the thyristors. , At the same time as the firing control of the fired thyristor is stopped, both the switches are selectively turned on instantly depending on whether the voltage of the power source is positive or negative, and the stored energy of the auxiliary tap and each capacitor is used to turn on the switches. An AC constant voltage device comprising: a commutation control circuit for forcibly commutating and extinguishing each of the thyristors. (2) A transformer connected to an AC power supply is provided with a plurality of output taps, one end of each of a plurality of bidirectional thyristors is connected to each of the output taps, and the other end of each of the thyristors is connected to an output terminal. , by tap switching control based on the detection of the voltage fluctuation of the power supply, the firing control of the already fired thyristor is stopped, and after the commutation of the thyristor is extinguished, the firing control of the thyristor of the target output tap is started; In an AC constant voltage device that supplies constant voltage alternating current to a load connected to an output terminal, a secondary winding is inserted between the connection point at the other end of each of the thyristors and the output terminal, and the a reactor for injecting commutation energy whose intermediate tap is grounded; two semiconductor switches for positive and negative commutation provided between the DC power source and both ends of the primary winding; and the already-ignited switch. At the same time as the ignition control of the thyristors is stopped, both the switches are selectively instantaneously turned on depending on whether the voltage of the AC power supply is positive or negative, and the induced energy of the secondary winding causes the thyristors to be forced into rotation. An AC constant voltage device characterized by comprising a commutation control circuit that extinguishes current arc.